Josephson break junction thin film device

ABSTRACT

A Josephson break junction device suitable for highly sensitive electronic detecting systems. A superconductor film such as YBa 2  Al 3  O 7  is deposited on a substrate such as a single-crystal MgO. The film is fractured across a narrow strip by at least one indentation in the substrate juxtaposed from the strip to form a break junction. A transducer is affixed to the substrate for applying a bending movement to the substrate to regulate the distance across the gap formed at the fracture to produce a Josephson turned junction effect. Alternatively, or in addition to the transducer, a bridge of a nobel metal is applied across the gap to produce a weak-link junction.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

The present invention relates generally to Josephson junctions inthin-film superconductors, and more particularly to Josephson junctionsin high temperature oxide superconductors suitable for use in variouselectronic components.

The application of superconductor devices has been largely limited toexperimentation because of the costly cryogenic requirements to operatebelow their transition temperatures, T_(c). However, a large number ofsuperconductors with high transition temperatures are now commerciallyavailable, the highest currently being close to 125° K. in a Tl-basedmaterial. This enables use of more economical cryogenic refrigerants,such as nitrogen which liquifies at 77 K.(-195.8 F.), and a potentialfor a developing superconducting electronic industry.

Common to all the high temperature oxide superconductors is the copperoxide (CuO) planes, which produces a layered structure with largeanisotropies in mechanical, electrical and thermal properties. They arebrittle, opaque, hard and, at room temperature, poor electricalconductors. Another common property of these materials is their shortaxis-dependent coherence length ξ, typically less than 50 Å.

YBa₂ Cu₃ O₇, herein after referred to as SC-123, has a superconductingtransition temperature T_(c) =92° K. and is one of the easiest toprepare in single phase in either bulk or thin-film form. Others, suchas Tl- and Bi-based oxide superconductors have higher transitiontemperatures but are more difficult to prepare in single-phase form dueto the proximity of several closely related structures with differenttransition temperatures.

The fundamental superccnductor device is the Josephson junction. Itconsists of two independent superconductors weakly coupled to each otherby a coupling structure. There are four main types of couplingstructures. These are the "classic" tunnel junction, layered structures,microbridges, and point contacts. To behave as a Josephson junction, thesize of the coupling structure must be of the order of the coherencelength ξ of the superconductors. The coherence length ξ of lowtemperature superconductors is several orders of magnitude larger thanthe crystal unit cell. This made it easy for fabrication of lowtemperature Josephson junction devices. In the new high temperaturesuperconductors, however, the coherence length ξ is of the order of thecrystal lattice spacing or smaller. This means that the couplingstructure must be of the order of a unit lattice spacing or less. Thesmallness of the coherence length has been at the root of the problem ofmaking a reliable Josephson junction.

Conventional methods of making Josephson junctions, such as multi-layerdeposition of a superconductor-normal-insulator-superconductor (SNIS),e.g. Nb-Al-A₂ O₃ -Nb, and microbridges, are not suitable for the newhigh temperature superconductors. The difficulty in making a microbridgejunction is in the patterning of lateral dimensions of the order of 30Å. Larger microbridges, on the order 1 μm (10,000 Å) have beenfabricated but the critical currents I_(c) are so high that the bridgesmelt when they become normal, i.e. non-superconductive. In the case ofSIS junctions, the coherence length ξ along the crystallographicc-direction is in the order of 5 Å, which is about one-third of the unitcell size in that direction. Therefore, it is very difficult to grow auniform thin layer of insulating material. There is also interdiffusionamong different layers, and the possibility of electrical shorts betweenlayers. Praseodymium (Pt) is the only rare earth element which can besubstituted for yttrium(Y) in SC-123, and that is not a superconductor.Several research groups have tried to use PrBa₂ Cu₃ O₇ to form a SISstructure because it has similar lattice parameters and thermalexpansion coefficients as SC-123, but so far they have not demonstratedJosephson junction effects.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aJosephson junction in a thin-film superconducting device for use invarious electronic components, sensors and the like requiring highsensitivity.

Still another object is to provide a reliable thin-film Josephsonjunction superconductor which can be easily manufactured with uniformquality and performance characteristics.

A further object is to provide a thin-film Josephson junction which issuitable for use as an active element in electronic circuits.

A further object is to fabricate a SQUID using Josephson junctionsformed by microindentation of a thin-film superconductor deposited on asubstrate.

A still further object is to provide a method for fabricating aJosephson junction thin-film superconductor devices for use incommunications, robotics, computers, sensors and the like.

Briefly, these and other objects and novel features of the invention areaccomplished by a superconductor film deposited on a substrate. The filmis fractured across a narrow strip of the film by at least oneindentation in the substrate and juxtaposed from the strip to form aso-called Josephson break junction. In one embodiment, a bending momentis applied to the substrate to regulate the distance across the gapformed at the fracture to produce a desired Josephson tunnel junctioneffect. In another embodiment, a bridge of a nobel metal across the gapprovides a weak-link junction effect.

The process for making the Josephson break junction includes patterningof a thin-film superconductor on a substrate, pressing indenters intothe substrate on at least one side of a narrow strip of thesuperconductor with an applied load sufficient to induce a definedfracture extending through the thin-film strip. A bending moment isapplied over the substrate to regulate the gap distance across thefracture to produce the desired Josephson junction effect.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a thin-film high temperature superconductingdevice including a single Josephson break junction according to theinvention;

FIG. 2 is a side view of the device of FIG. 1;

FIG. 3 is a further enlarged fragmental view in cross section of thedevice of FIG. 1 at the junction taken in a plane along the line 3--3 ofFIG. 1;

FIG. 4 is a fragmental view in cross section of the device taken in aplane through the Josephson junction along the line 4--4 of FIG. 3;

FIG. 5 is an electrical schematic of a feedback circuit for regulatingthe Josephson junction effect of the device of FIG. 1;

FIG. 6 represents a view in cross section like FIG. 3 of the device witha bending moment applied by the circuit of FIG. 5;

FIG. 7 is a side view of an alternative embodiment according to theinvention of the device of FIG. 3 utilizing a bridge for regulating theJosephson junction effect;

FIG. 8 is a block diagram of a process according to the invention forfabricating the devices of FIGS. 1;

FIG. 9 is a perspective view of an indenter head juxtaposed above anindentation in a substrate of the device of FIG. 1;

FIG. 10 is a perspective view of another embodiment of a thin-filmsuperconducting device according to the invention with two Josephsonbreak junctions located in a superconducting quantum interference device(SQUID) configuration; and

FIG. 11 is an electrical schematic of a magnetic field detectorutilizing the SQUID of FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings wherein like reference charactersdesignate like or corresponding parts throughout the several views,FIGS. 1 and 2 illustrate one embodiment of a single Josephson breakjunction thin-film high temperature superconducting device 10 accordingto the invention. The device comprises a thin-film 12 of hightemperature SC-123, on the order of 0.1 μm thickness deposited on oneside of a generally planar, single-crystal MgO substrate 14, having asimilar lattice parameters at their interface. That is, in the a-b planedefined by arrows a and b of FIGS. 1 and 9, the inter-atomic spacings ofthe substrate and the thin-film are approximately equal. Consequently,as is well known, the thin-film will grow epitaxially with the samecrystallographic orientation as that of the substrate. Low temperaturesuperconductors, however, would require no similarity in latticestructures with the substrate. The thickness of thin-film 12 isdetermined according to the ease by which a crack will propagate. Othercombinations of high temperature oxide superconductors, such as the Biand Tl-based superconducting compounds, with single-crystal substratessuch as SrTiO₃, LaAlO₃ NdGaO₃ and GaAlO₃, and low temperaturesuperconductors such as Nb, Pb, Al with any standard substrate, arecontemplated. The thin-film superconductors exhibit very small coherencelengths ξ, around 5 Å, in the crystallographic c-direction and criticalcurrent densities J_(c) larger than 10⁶ Å/cm² at 77° K. with littledegradation in fields up tip 5 Tesla. Thin-film 12 defines an elongatestrip 12a aligned generally with an axis X-X which, for the MgOsubstrate 14, is 45° from the crystallographic a and b directions. Theopposite side of substrate 14 is contiguously bonded to a transducer 16,such as a piezoelectric bimorph of laminated piezoelectric plates ofopposite polarity, quartz, to produce a bending moment in substrate 14about the axis Y-Y when a voltage is applied at terminals 17a and 17b.

Thin-film strip 12a includes neck sections 18a and 18b, of equal widthsin the order of 0.1 to 1.0 μm. The width is determined to insure acomplete crack across neck 18a. Indentations 20 in the surface ofsubstrate 14, on opposite sides of neck 18a, along the axis Y-Y initiatefractures in a secondary cleavage plane d 45° from the crystallographica-c and b-c planes. As shown in FIG. 4, indentations 20 initiatefractures represented by the dash lines in cleavage plane d lyingperpendicular to the substrate surface and intersecting that surfacealong a line 45° to the direction of arrow a (see FIG. 9). For othersubstrate the directions a crack will take must be determined beforeindentation. The fractures grow until they overlap under thin-film neck18a and form a permanent crack 22 extending through thin-film 12. Thefracture also extends through substrate 14 and across neck 18a to form aJosephson break junction at gap G.

The separation distance of opposed surfaces in gap G in SC-123 istypically about 1000 Å with an unstressed MgO substrate 14. To operateas a Josephson junction with SC-123 material, gap G must be typically 50Å separation, which is the coherence length in the a-b plane of SC-123.At this separation, the junction is of the Josephson type and is mostsensitive for measuring magnetic fields, infrared radiation, etc. Abending moment is therefore applied to substrate 14 by applying anelectric potential across transducer 16 to produce this optimum crackseparation.

Referring to FIG. 5, device 10, within a cryogenic chamber not shown, isconnected in a feedback control circuit for maintaining the desiredwidth of gap G. A d.c. bias current, applied through neck 18a from apower supply 28 and in series with a potentiometer 30 and current meter32, is connected to thin-film terminals 34a and 34b at opposite sides ofgap G. The voltage across gap G produced by the bias current ismodulated by an a. c. signal applied to transducer 16 through acapacitor 46 and is detected at terminals. 36a and 36b by a lock-inamplifier 38. A feedback signal is connected to transducer 16 through anamplifier 40 and resistor 42 and terminal 17a, the other terminal 17b,being connected to ground. A voltmeter 48, may be connected acrossresister 42 to measure the voltage which is useful for calibrationpurposes.

In the absence of a Josephson junction, i.e. no gap, the criticalcurrent I_(c) is maximum. However, a Josephson junction provides theeffect of a decreasing critical current I_(c) with increasingseparation. Beyond a certain separation the critical current I_(c) dropsto zero.

Therefore, a large separation in gap, as occurs when the substrate isfractured in the above manner, must be reduced in order to reach theoptimal critical current I_(c). FIG. 6 is an exaggerated representationof gap G with a feedback signal to transducer 16 producing a bendingmoment reducing the gap. For example, a gap G of 1,000 Å (FIG. 3) isreduced to approximately 50 Å (FIG. 6) by a bending moment applied bytransducer 16.

The pattern of the thin-film shown in FIG. 1 is particularly suitablefor measuring the critical current density J_(c) of the film. Neck 18bis of the same width and thickness as neck 18a but has no gap. Thusly,the critical current density J_(c) of the film with no gap can beobserved together with the I-V characteristics of the junction at gap G.In this way values of the density J_(c) of the junction in neck 18anormalized to the critical current densities J_(c) 's of thesuperconductor at 77° K. may be correlated to the crack face separation,i.e. the distance between the opposed surfaces of gap G as well asmagnetic field effects in terms of interference effects.

FIG. 7 represents an alternative embodiment of a thin-filmsuperconducting device in which a high temperature superconductingthin-film 54 is deposited on a substrate 56 in a pattern similar to thepattern of FIG. 1 with a gap formed by a crack in the substrate. Thematerials are as described for device 10 of FIG. 1. A Josephson junction58 is formed in thin-film 54 by a bridge 60 of a controlled amount of anoble metal or alloy such as gold, silver or platinum creating thereby aweak-link junction. For example, a bridge 60 of gold across a gapenlarges the effective coherent length ξ from 50 Å to approximately 1000Å.

Methods of fabricating the above-described Josephson break junctionthin-film superconducting devices are best described with reference toFIGS. 8 and 10. Starting with a blank single crystal substrate 14 suchas MgO, SrTiO₃ or LaAlO₃, a thin-film 12 of oxide superconductingmaterial, such as SC-123, is grown on a planar surface of the substrateby a conventional deposition process such as laser ablation, sputtering,chemical vapor deposition or the like, to a thickness of around 0.1 to1.0 μm. The thin-film is patterned using any technique such asphotolithography, laser ablation, masking, etc., to define an elongatestrip with a narrow neck section intermediate the ends of around 1.0 to10.0 μm width. Utilizing conventional microindentation techniques, twosmall indentations 20 are made in the substrate on opposite sides ofneck section 18a on a line d, normal to the length of the strip, by aninverted pyramidal diamond wedge 64 with equal sides (FIG. 9). One pairof diagonally opposed vertices of wedge 64 lie on line d for ease offracturing the substrate crystal. The loading and penetration depth ofwedge 64 is carefully controlled to ensure propagation of a completefracture of the substrate beneath the neck section of the superconductorand a break in the superconductor at the fracture. The position, depthand applied load for indentation, and other wedge configurations, aredetermined in order to guarantee a reproducible sharp break. If thesubstrate fractures induced by the indentations do not overlap, anadditional bending strain may be applied to the substrate to spread thefracture until there is sufficient overlap. Other wedge configurationsare contemplated to provide a sharp crack.

In the embodiment of FIG. 2, piezoelectric transducer 16 is contiguouslyfixed to the substrate 14 on the surface opposite of thin-film 12 andcauses the substrate to bend with an applied voltage for establishingthe gap separation necessary for optimal critical current I_(c). In theembodiment of FIG. 7, in addition to attaching substrate 56 to atransducer, a bridge 60 of gold is applied across the gap 58 toestablish a weak-link junction for this optimum critical current I_(c).

Josephson break junctions according to the invention as applied to asuperconducting quantum interference device [SQUID] are particularlysuitable for measurement of very small magnetic fields, currents andvoltages. FIG. 10 illustrates a DC SQUID 65 in which two Josephsonjunctions 66a and 66b are formed in a superconductor ring configuration.A superconducting thin-film 68 is deposited on a substrate 70 in apattern having a bifurcated center section conforming two parallel necks72a and 72b. Indentations 74 on opposite sides of the necks are alignedto produce a fracture 76 in substrate 70 and gaps 66a and 66b inthin-film necks 72a and 72b.

Referring to FIG. 11, SQUID 65 with a d.c. bias supply 67 is shown in acircuit configuration for measuring an electromagnetic field B. Thevoltage drop across the SQUID 65 is detected through a step-uptransformer 78 and lock-in amplifier 80. An a.c. reference signal atterminal 82 is fed through capacitor 83 to a field coil 84 in magneticproximity to SQUID 65 to modulate the voltage drop across the SQUID. Thed.c. output 86 at lock-in amplifier 80 is fed back to field coil 84 viad.c. amplifier 88 and resistor 90 to control field intensity. Thevoltage across resistor 90, as measured by voltmeter 92, is directlyproportional to the external magnetic field B.

Some of the many advantages and novel features of the invention shouldnow be readily apparent. A Josephson junction can be made usingindentation techniques and by taking advantage of the unique cleavageproperties of various substrate materials and the large coherence lengthof the superconductor. Cracks are introduced in the superconductors byinducing a fracture in the substrate. The Josephson junction iscontrollable through bending the substrate. The methods of making thedevice is very simple and has potential for use in large scaleintegration.

Many modifications and variations of the present invention are possiblein view of the above disclosure. It is therefore to be understood, thatwithin the scope of the appended claims, the invention may be practicedotherwise than as specifically described.

We claim:
 1. A Josephson junction device comprising:a substrate havingorthogonal crystallographic a-b, a-c and b-c planes and a predeterminedcleavage plane; a thin-film strip of an oxide superconductor on saidsubstrate, a c-axis of said oxide superconductor is perpendicular to thea-b plane of the substrate and an orientation of a crystallographic a-bplane of said oxide superconductor and an orientation of the a-b planeof the substrate are substantially the same; said strip having afracture substantially along said cleavage plane defining a gap in saidstrip; and means for regulating a separation distance between oppositesurfaces of the gap for achieving an optimum current density at saidgap.
 2. A device according to claim 1 wherein:said transducer meansincludes a piezoelectric element contiguously affixed to said substrate.3. A device according to claim 2 wherein:said transducer is apiezoelectric bimorph having laminated piezoelectric plates of oppositepolarity.
 4. A Josephson junction device comprising:a substrate havingorthogonal crystallographic a-b, a-c and b-c planes and a predeterminedcleavage plane; a thin-film strip of an oxide superconductor on saidsubstrate, a c-axis of said oxide superconductor is perpendicular to thea-b plane of the substrate and an orientation of a crystallographic a-bplane of said oxide superconductor and an orientation of the a-b planeof the substrate are substantially the same; said strip having afracture substantially along said cleavage plane defining a gap in saidstrip; and means for regulating a separation distance between oppositesurfaces of the gap for achieving an optimum current density at saidgap, said regulating means further including a transducer means forapplying a bending moment to said substrate in a plane across said gap.5. A device according to claim 4 further comprising:means for applying ad.c. bias current through said strip for producing a voltage across saidgap; means for detecting the voltage; and means for applying a feedbacksignal to said element for bending said substrate.
 6. A Josephson breakjunction device comprising:a substrate having orthogonalcrystallographic a-b, a-c and b-c planes, and a predetermined cleavageplane; a thin-film strip of an oxide superconductor material on sadsubstrate, a c-axis of said oxide superconductor is perpendicular to thea-b plane of the substrate and an orientation of a crystallographic a-bplane of said oxide superconductor and an orientation of the a-b planeof the substrate are substantially the same; said strip having afracture substantially along said cleavage plane defining a gap withopposed faces in said strip; an electrical power source connected tosaid strip for conducting a direct current across said gap; anelectromechanical transducer contiguously affixed to said substrate forproducing a bending moment in a plane across said gap for regulating thedistance between the opposed faces of said gap; a lock-in amplifierconnected to said strip for detecting a voltage across the gap; andfeedback means connected between said lock-in amplifier and saidtransducer for electrically energizing said transducer as a function ofthe voltage.
 7. A Josephson junction device comprising:a substratehaving orthogonal crystallographic a-b, a-c and b-c planes, and apredetermined cleavage plane; a thin-film strip of an oxidesuperconductor material epitaxially deposited on said substrate, ac-axis of said oxide superconductor is perpendicular to the a-b plane ofthe substrate and an orientation of a crystallographic a-b plane of saidoxide superconductor and an orientation of the a-b plane of thesubstrate are substantially the same, said strip including a fracturewith opposed faces substantially along the cleavage plane; and atransducer means contiguously affixed to said substrate for producing abending moment in a plane across said fracture for regulating thedistance between said faces.
 8. A Josephson junction device,comprising:a substrate having orthogonal crystallographic a-b, a-c andb-c planes and a predetermined cleavage plane; a thin-film strip of anoxide superconductor on said substrate, a c-axis of said oxidesuperconductor is perpendicular to the a-b plane of the substrate and anorientation of a crystallographic a-b plane of said oxide superconductorand an orientation of the a-b plane of the substrate are substantiallythe same, said strip has a superconducting transition temperature aboveapproximately 30 K.; said strip having a fracture substantially alongsaid cleavage plane defining a gap in said strip; and means forregulating a separation distance between opposite surfaces of the gapfor achieving an optimum current density at said gap.
 9. A Josephsonjunction device, comprising:a substrate having orthogonalcrystallographic a-b, a-c and b-c planes and a predetermined cleavageplane, said substrate is selected from a group consisting of MgO,SrTiO₃, NdGaO₃, GaAlO₃, and LaAlO₃ ; a thin-film strip of an oxidesuperconductor on said substrate, a c-axis of said oxide superconductoris perpendicular to the a-b plane of the substrate and an orientation ofa crystallographic a-b plane of said oxide superconductor and anorientation of the a-b plane of the substrate are substantially thesame, said oxide superconductor is selected from the group consisting ofYBa₂ Cu₃ O₇, and Bi and Ti families; said strip having a fracturesubstantially along said cleavage plane defining a gap in said strip;and means for regulating a separation distance between opposite surfacesof the gap for achieving an optimum current density at said gap.
 10. AJosephson junction device, comprising:a substrate having orthogonalcrystallographic a-b, a-c and b-c planes and a predetermined cleavageplane; a thin-film strip of an oxide superconductor on said substrate, ac-axis of said oxide superconductor is perpendicular to the a-b plane ofthe substrate and an orientation of a crystallographic a-b plane of saidoxide superconductor and an orientation of the a-b plane of thesubstrate are substantially the same; sad strip having a fracturesubstantially along said cleavage plane defining a gap in said strip;means for regulating a separation distance between opposite surfaces ofthe gap for achieving an optimum current density at said gap; and aconductor means bridging said gap for increasing a coherence length insaid oxide superconductor.
 11. A device according to claim 1wherein:said cleavage plane is 45° from the crystallographic a-c planeof the substrate.